Identification of broadly reactive DR restricted epitopes

ABSTRACT

The present invention is based on peptide binding specificities of HLA DRB1*0401, DR1 and DR7. Peptides binding to these DR molecules have a motif characterized by a large aromatic or hydrophobic residue in position 1 (Y, F, W, L, I, V, M) and a small, non charged residue in position 6 (S, T, C, A, P, V, I, L, M). In addition, allele-specific secondary effects and secondary anchors are defined, and these results were utilized to derive allele specific algorithms. By the combined use of such algorithms peptides capable of degenerate DR1, 4, 7 binding were identified.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. application Ser. No. 09/009,953 filed Jan. 21, 1998, which claims the benefit of the Feb. 7, 1997 filing date of U.S. Application Ser. No. 60/037,432 and claims the benefit of the Jan. 23, 1997 filing date of U.S. Application Ser. No. 60/036,713. Each of these applications is incorporated herein by reference in its entirety.

BACKGROUND

[0002] Helper T lymphocytes (HTL) play several important functions in immunity to pathogens. First, they provide help for induction of both CTL and antibody responses. By both direct contact and by secreting lymphokines such as IL2 and IL4, HTL promote and support the expansion and differentiation of T and B cell precursors into effector cells. In addition, HTL can also be effectors in their own right, an activity also mediated by direct cell contact and secretion of lymphokines, such as IFNγ and TNFα HTL have been shown to have direct effector activity in tumors, as well as viral, bacterial, parasitic, and fungal infections.

[0003] HTL recognize a complex formed between Class II MHC molecules and antigenic peptides, usually between 10 and 20 amino acids long, with an average size of between 13 and 16 amino acids. Peptide-Class II interactions have been analyzed in detail, both at the structural and functional level, and peptide motifs specific for various human and mouse Class II molecules have been proposed.

[0004] In the last few years, epitope based vaccines have received considerable attention as a possible means for developing novel prophylactic vaccines and immunotherapeutic strategies. Selection of appropriate T and B cell epitopes should focus an immune response towards conserved epitopes of pathogens which are characterized by high sequence variability (such as HIV, HCV and Malaria).

[0005] In addition, focusing an immune response towards selected determinants could be of value in the case of various chronic viral diseases and cancer, where T cells directed against the immunodominant epitopes might have been inactivated while T cells specific for subdominant epitopes might have escaped T cell tolerance. The use of epitope based vaccines also avoids “suppressive” T cell determinants which can induce TH₂ responses in conditions where a TH₁ response is desirable, or vice versa.

[0006] Finally, epitope based vaccines also offer the opportunity to include in the vaccine construct epitopes that have been engineered for modulated potency, by increasing MHC binding affinity, by altering its TCR contact residues, or both. Inclusion of completely synthetic non-natural epitopes or epitopes genetically unrelated to the targeted pathogen or epitope (such as TT derived “universal” epitopes), may also modulate the HTL response towards a TH₁, or TH₂ phenotype.

[0007] Once appropriate epitope determinants have been defined, they can be assorted and delivered by various means, which include lipopeptides, viral delivery vectors, particles of viral or synthetic origin, naked or particle absorbed cDNA.

[0008] However, before appropriate epitopes can be defined, one major obstacle has to be overcome, namely the very high degree of polymorphism of the MHC molecules expressed in the human population. In fact, more than two hundred different types of HLA Class I and Class II molecules have already been identified. It has been demonstrated that in the case of HLA Class I molecules, peptides capable of binding several different HLA Class I molecules can be identified. Over 60% of the known HLA Class I molecules can, in fact, be grouped in four broad HLA supertypes, characterized by similar peptide binding specificities (HLA supermotifs).

[0009] The present invention addresses these and other needs.

SUMMARY

[0010] The present invention is based, at least in part, on the discovery and validation of specific motifs and assay systems for various DR molecules that are representative of DR distribution in the worldwide population. The application of these motifs and systems for the identification of broadly degenerate HLA Class II binding peptides is also described.

Definitions

[0011] The term “peptide” is used interchangeably with “oligopeptide” in the present specification to designate a series of residues, typically L-amino acids, connected one to the other typically by peptide bonds between the alpha-amino and carbonyl groups of adjacent amino acids. The oligopeptides of the invention are less than about 50 residues in length and usually consist of between about 10 and about 30 residues, more usually between about 12 and 25, and often 15 and about 20 residues.

[0012] An “immunogenic peptide” is a peptide which comprises an allele-specific motif such that the peptide will bind an MHC molecule and induce a HTL response. Immunogenic peptides of the invention are capable of binding to an appropriate HLA molecule and inducing an HTL response against the antigen from which the immunogenic peptide is derived.

[0013] A “conserved residue” is a conserved amino acid occupying a particular position in a peptide motif, typically one where the MHC structure may provide a contact point with the immunogenic peptide. One to three, typically two, conserved residues within a peptide of defined length defines a motif for an immunogenic peptide. These residues are typically in close contact with the peptide binding groove, where their side chains may be buried in specific pockets of the groove itself.

[0014] The term “motif” refers to a pattern of residues of defined length, usually between about 8 to about 11 amino acids, which is recognized by a particular MHC allele. The term “supermotif” refers to motifs that, when present in an immunogenic peptide, allow the peptide to bind more than one HLA antigen. The supermotif preferably is recognized by at least one HLA allele having a wide distribution in the human population, preferably recognized by at least two alleles, more preferably recognized by at least three alleles, and most preferably recognized by more than three alleles.

[0015] The phrases “isolated” or “biologically pure” refer to material which is substantially or essentially free from components which normally accompany it in its native state. Thus, the peptides of this invention do not contain materials normally associated with their in situ environment, e.g., MHC I molecules on antigen presenting cells. Even where a protein has been isolated to a homogenous or dominant band, there may be trace contaminants in the range of 5-10% of native protein which co-purify with the desired protein. Isolated peptides of this invention do not contain such endogenous co-purified protein.

[0016] The term “residue” refers to an amino acid or amino acid mimetic incorporated in an oligopeptide by an amide bond or amide bond mimetic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 presents a map of the positive or negative effect of each of the 20 naturally occurring amino acids on DRB1*0401 (DR4w4) binding capacity when occupying a particular position, relative to the main P1-P6 anchors.

[0018]FIG. 2A presents a map of the positive or negative effect of each of the 20 naturally occurring amino acids on DR1 (DRB1*0101) binding capacity when occupying a particular position, relative to the main P1-P6 anchors.

[0019]FIG. 2B presents a map of the positive or negative effect of each of the 20 naturally occurring amino acids on DR7 (DRB1*0701) binding capacity when occupying a particular position, relative to the main P1-P6 anchors.

[0020]FIG. 3 summarizes features of the DR binding studies described herein. Included is identification of the radiolabled probes utilized, the specific alleles studied for each DR antigen, and the cell line utilized as the source of purified MHC. Also listed are alternative names for each molecule that are commonly utilized in the literature. Nomenclature for the various alleles is used interchangeably herein.

[0021]FIG. 4A and FIG. 4B depict results from applying different cut-off values to the DRB1*0401 algorithm described herein.

[0022]FIG. 5 depicts results of applying different cut-off values to the DR 1-4-7 algorithm described herein.

[0023]FIG. 6 shows phenotypic frequencies of ten prevalent HLA-DR antigens across five ethnic populations.

[0024]FIG. 7A and FIG. 7B show the binding capacities of various peptides to different DR molecules.

[0025]FIG. 8 summarizes binding capacities of degenerate peptides to a panel of twelve common DR types.

[0026]FIG. 9 compares the frequency with which DR 1-4-7 degenerate peptides and non-degenerate peptides bind different DR types.

[0027]FIG. 10 and FIG. 11 show predicted DR 1-4-7 degenerate peptides derived from different organisms.

DESCRIPTION OF THE EMBODIMENTS

[0028] The present invention relates to compositions and methods for preventing, treating or diagnosing a number of pathological states such as viral, fungal, bacterial and parasitic diseases and cancers. In particular, it provides novel peptides capable of binding selected major histocompatibility complex (MHC) class II molecules and inducing an immune response.

[0029] The peptides described here can also be used as HTL peptides in combination with peptides which induce a CTL response. This combination is described in WO 95/07077.

[0030] The DR-binding peptides of the present invention or nucleic acids encoding them can be administered to mammals, particularly humans, for prophylactic and/or therapeutic purposes. The DR peptides can be used to enhance immune responses against other immunogens administered with the peptides. For instance, CTL/DR mixtures may be used to treat and/or prevent viral infection and cancer. Alternatively, immunogens which induce antibody responses can be used. Examples of diseases which can be treated using the immunogenic mixtures of DR peptides and other immunogens include prostate cancer, hepatitis B, hepatitis C, AIDS, renal carcinoma, cervical carcinoma, lymphoma, CMV and condyloma acuminatum.

[0031] The DR-binding peptides or nucleic acids encoding them may also be used to treat a variety of conditions involving unwanted T cell reactivity. Examples of diseases which can be treated using DR-binding peptides include autoimmune diseases (e.g., rheumatoid arthritis, multiple sclerosis, and myasthenia gravis), allograft rejection, allergies (e.g., pollen allergies), lyme disease, hepatitis, LCMV, post-streptococcal endocarditis, or glomerulonephritis, and food hypersensitivities.

[0032] In therapeutic applications, the immunogenic compositions or the DR-binding peptides or nucleic acids of the invention are administered to an individual already suffering from cancer, autoimmune disease, or infected with the virus of interest. Those in the incubation phase or the acute phase of the disease may be treated with the DR-binding peptides or immunogenic conjugates separately or in conjunction with other treatments, as appropriate.

[0033] In therapeutic applications, compositions comprising immunogenic compositions are administered to a patient in an amount sufficient to elicit an effective CTL response to the virus or tumor antigen and to cure or at least partially arrest symptoms and/or complications. Similarly, compositions comprising DR-binding peptides are administered in an amount sufficient to cure or at least partially arrest the symptoms of the disease and its complications. An amount adequate to accomplish these uses is defined as “therapeutically effective dose.” Amounts effective for these uses will depend on, e.g., the peptide composition, the manner of administration, the stage and severity of the disease being treated, the weight and general state of health of the patient, and the judgment of the prescribing physician.

[0034] Therapeutically effective amounts of the immunogenic compositions of the present invention generally range for the initial immunization (that is for therapeutic or prophylactic administration) from about 1.0 μg to about 10,000 μg of peptide for a 70 kg patient, usually from about 100 to about 8000 μg, and preferably between about 200 and about 6000 μg. These doses are followed by boosting dosages of from about 1.0 μg to about 1000 μg of peptide pursuant to a boosting regimen over weeks to months depending upon the patient's response and condition by measuring specific CTL activity in the patient's blood.

[0035] It must be kept in mind that the compositions of the present invention may generally be employed in serious disease states, that is, life-threatening or potentially life-threatening situations. In such cases, in view of the minimization of extraneous substances and the relative nontoxic nature of the conjugates, it is possible and may be felt desirable by the treating physician to administer substantial excesses of these compositions.

[0036] For prophylactic use, administration should be given to risk groups. For example, protection against malaria, hepatitis, or AIDS may be accomplished by prophylactically administering compositions of the invention, thereby increasing immune capacity. Therapeutic administration may begin at the first sign of disease or the detection or surgical removal of tumors or shortly after diagnosis in the case of acute infection. This is followed by boosting doses until at least symptoms are substantially abated and for a time period thereafter. In chronic infection, loading doses followed by boosting doses may be required.

[0037] Treatment of an infected individual with the compositions of the invention may hasten resolution of the infection in acutely infected individuals. For those individuals susceptible (or predisposed) to developing chronic infection the compositions are particularly useful in methods for preventing the evolution from acute to chronic infection. Where the susceptible individuals are identified prior to or during infection, for instance, as described herein, the composition can be targeted to them, minimizing need for administration to a larger population.

[0038] The peptide mixtures or conjugates can also be used for the treatment of chronic infection and to stimulate the immune system to eliminate virus-infected cells in carriers. It is important to provide an amount of immuno-potentiating peptide in a formulation and mode of administration sufficient to effectively stimulate a cytotoxic T cell response. Thus, for treatment of chronic infection, a representative dose is in the range of about 1.0 μg to about 5000 μg, preferably about 5 μg to 1000 μg for a 70 kg patient per dose. Immunizing doses followed by boosting doses at. established intervals, e.g., from one to four weeks, may be required, possibly for a prolonged period of time to effectively immunize an individual. In the case of chronic infection, administration should continue until at least clinical symptoms or laboratory tests indicate that the viral infection has been eliminated or substantially abated and for a period thereafter.

[0039] The pharmaceutical compositions for therapeutic or prophylactic treatment are intended for parenteral, topical, oral or local administration. Typically, the pharmaceutical compositions are administered parenterally, e.g., intravenously, subcutaneously, intradermally, or intramuscularly. Because of the ease of administration, the vaccine compositions of the invention are particularly suitable for oral administration. Thus, the invention provides compositions for parenteral administration which comprise a solution of the peptides or conjugates dissolved or suspended in an acceptable carrier, preferably an aqueous carrier. A variety of aqueous carriers may be used, e.g., water, buffered water, 0.9% saline, 0.3% glycine, hyaluronic acid and the like. These compositions may be sterilized by conventional, well known sterilization techniques, or may be sterile filtered. The resulting aqueous solutions may be packaged for use as is, or lyophilized, the lyophilized preparation being combined with a sterile solution prior to administration. The compositions may contain pharmaceutically acceptable auxiliary substances as required to approximate physiological conditions, such as pH adjusting and buffering agents, tonicity adjusting agents, wetting agents and the like, for example, sodium acetate, sodium lactate, sodium chloride, potassium chloride, calcium chloride, sorbitan monolaurate, triethanolamine oleate, and other agents.

[0040] The concentration of DR and/or CTL stimulatory peptides of the invention in the pharmaceutical formulations can vary widely, i.e., from less than about 0.1%, usually at or at least about 2% to as much as 20% to 50% or more by weight, and will be selected primarily by fluid volumes, viscosities, and other parameters, in accordance with the particular mode of administration selected.

[0041] The peptides and conjugates of the invention may also be administered via liposomes, which serve to target the conjugates to a particular tissue, such as lymphoid tissue, or targeted selectively to infected cells, as well as increase the half-life of the peptide composition. Liposomes include emulsions, foams, micelles, insoluble monolayers, liquid crystals, phospholipid dispersions, lamellar layers and the like. In these preparations the peptide to be delivered is incorporated as part of a liposome, alone or in conjunction with a molecule which binds to, e.g., a receptor prevalent among lymphoid cells, such as monoclonal antibodies which bind to the CD45 antigen, or with other therapeutic or immunogenic compositions. Thus, liposomes filled with a desired peptide or conjugate of the invention can be directed to the site of lymphoid cells, where the liposomes then deliver the selected therapeutic/immunogenic peptide compositions. Liposomes for use in the invention are formed from standard vesicle-forming lipids, which generally include neutral and negatively charged phospholipids and a sterol, such as cholesterol. The selection of lipids is generally guided by consideration of, e.g., liposome size, acid lability and stability of the liposomes in the blood stream. A variety of methods are available for preparing liposomes, as described in, e.g., Szoka, et al., Ann. Rev. Biophys. Bioeng. 9, 467 (1980), U.S. Pat. Nos. 4,235,871, 4,501,728, 4,837,028, and 5,019,369, incorporated herein by reference.

[0042] For targeting to immune cells, a ligand to be incorporated into the liposome can include, e.g., antibodies or fragments thereof specific for cell surface determinants of the desired immune system cells. A liposome suspension containing a peptide or conjugate may be administered intravenously, locally, topically, and by other modes of administration in a dose which varies according to, inter alia, the manner of administration, the conjugate being delivered, and the stage of the disease being treated.

[0043] Alternatively, DNA or RNA encoding one or more DR peptides and a polypeptide containing one or more CTL epitopes or antibody inducing epitopes may be introduced into patients to obtain an immune response to the one or more polypeptides which the nucleic acid encodes. Wolff, et. al., Science 247: 1465-1468 (1990) describes the use of nucleic acids to produce expression of the genes which the nucleic acids encode. Such use is also disclosed in U.S. Pat. Nos. 5,580,859 and 5,589,466.

[0044] For solid compositions, conventional nontoxic solid carriers may be used which include, for example, pharmaceutical grades of mannitol, lactose, starch, magnesium stearate, sodium saccharin, talcum, cellulose, glucose, sucrose, magnesium carbonate, and the like. For oral administration, a pharmaceutically acceptable nontoxic composition is formed by incorporating any of the normally employed excipients, such as those carriers previously listed, and generally 10% to 95% of active ingredient, that is, one or more conjugates of the invention, and more preferably at a concentration of 25% to 75%.

[0045] For aerosol administration, the peptides are preferably supplied in finely divided form along with a surfactant and propellant. Typical percentages of conjugates are 0.01% to 20% by weight, preferably 1% to 10%. The surfactant must, of course, be nontoxic, and preferably soluble in the propellant. Representative of such agents are the esters or partial esters of fatty acids containing from 6 to 22 carbon atoms, such as caproic, octanoic, lauric, palmitic, stearic, linoleic, linolenic, olesteric and oleic acids with an aliphatic polyhydric alcohol or its cyclic anhydride. Mixed esters, such as mixed or natural glycerides may be employed. The surfactant may constitute 0.1% to 20% by weight of the composition, preferably 0.25% to 5%. The balance of the composition is ordinarily propellant. A carrier can also be included, as desired, as with, e.g., lecithin for intranasal delivery.

[0046] In another aspect the present invention is directed to vaccines which contain as an active ingredient an immunogenically effective amount of an immunogenic HTL epitope or HTL/CTL epitope conjugate or nucleic acid encoding them as described herein. The conjugate(s) may be introduced into a host, including humans, linked to its own carrier or as a homopolymer or heteropolymer of active peptide units. Such a polymer has the advantage of increased immunological reaction and, where different peptides are used to make up the polymer, the additional ability to induce antibodies and/or CTLs that react with different antigenic determinants of the virus or tumor cells. Useful carriers are well known in the art, and include, e.g., thyroglobulin, albumins such as bovine serum albumin, tetanus toxoid, polyamino acids such as poly(lysine/glutamic acid), hepatitis B virus core protein, hepatitis B virus recombinant vaccine and the like. The vaccines can also contain a physiologically tolerable (acceptable) diluent such as water, phosphate buffered saline, or saline, and further typically include an adjuvant. Adjuvants such as incomplete Freund's adjuvant, aluminum phosphate, aluminum hydroxide, or alum are materials well known in the art. And, as mentioned above, CTL responses can be primed by conjugating peptides of the invention to lipids, such as P₃CSS. Upon immunization with a peptide composition as described herein, via injection, aerosol, oral, transdermal or other route, the immune system of the host responds to the vaccine by producing large amounts of CTLs specific for the desired antigen, and the host becomes at least partially immune to later infection, or resistant to developing chronic infection.

[0047] Vaccine compositions containing the DR peptides of the invention are administered to a patient susceptible to or otherwise at risk of disease, such as viral infection or cancer to elicit an immune response against the antigen and thus enhance the patient's own immune response capabilities, for instance with CTL epitopes. Such an amount is defined to be an “immunogenically effective dose. ” In this use, the precise amounts again depend on the patient's state of health and weight, the mode of administration, the nature of the formulation, for example, but generally range from about 1.0 μg to about 5000 μg per 70 kilogram patient, more commonly from about 10 μg to about 500 μg per 70 kg of body weight.

[0048] In some instances it may be desirable to combine the peptide vaccines of the invention with vaccines which induce neutralizing antibody responses to the virus of interest, particularly to viral envelope antigens. For instance, PADRE® peptides can be combined with hepatitis vaccines to increase potency or broaden population coverage. Suitable hepatitis vaccines that can be used in this manner include, RECOMBIVAX HB® (Merck) and ENGERIX-B® (Smith-Kline).

[0049] For therapeutic or immunization purposes, the peptides of the invention can also be expressed by attenuated viral hosts, such as vaccinia or fowlpox. This approach involves the use of vaccinia virus as a vector to express nucleotide sequences that encode the peptides of the invention. Upon introduction into an acutely or chronically infected host or into a non-infected host, the recombinant vaccinia virus expresses the immunogenic peptide, and thereby elicits a host CTL response. Vaccinia vectors and methods useful in immunization protocols are described in, e.g., U.S. Pat. No. 4,722,848, incorporated herein by reference. Another vector is BCG (Bacille Calmette Guerin). BCG vectors are described in Stover et al., Nature 351, 456-460 (1991)) which is incorporated herein by reference. A wide variety of other vectors useful for therapeutic administration or immunization of the peptides of the invention, e.g., Salmonella typhi vectors and the like, will be apparent to those skilled in the art from the description herein.

[0050] Antigenic conjugates may be used to elicit CTL ex vivo, as well. The resulting CTL can be used to treat chronic infections (viral or bacterial) or tumors in patients that do not respond to other conventional forms of therapy, or will not respond to a peptide vaccine approach of therapy. Ex vivo CTL responses to a particular pathogen (infectious agent or tumor antigen) are induced by incubating in tissue culture the patient's CTL precursor cells (CTLp) together with a source of antigen-presenting cells (APC) and the appropriate immunogenic peptide. After an appropriate incubation time (typically 1-4 weeks), in which the CTLp are activated and mature and expand into effector CTL, the cells are infused back into the patient, where they will destroy their specific target cell (an infected cell or a tumor cell).

[0051] The peptides of this invention may also be used to make monoclonal antibodies. Such antibodies may be useful as potential diagnostic or therapeutic agents.

[0052] The peptides may also find use as diagnostic reagents. For example, a peptide of the invention may be used to determine the susceptibility of a particular individual to a treatment regimen which employs the peptide or related peptides, and thus may be helpful in modifying an existing treatment protocol or in determining a prognosis for an affected individual. In addition, the peptides may also be used to predict which individuals will be at substantial risk for developing chronic infection.

[0053] The following example is offered by way of illustration, not by way of limitation.

EXAMPLE Materials and Methods Cells

[0054] The following Epstein-Barr virus (EBV) transformed homozygous cell lines were used as sources of human HLA Class II molecules: LG2 [DRB1*0101 (DR1)]; GM3107 [DRB5*0101 (DR2w2a)]; MAT [DRB1*0301 (DR3)]; PREISS [DRB1*0401 (DR4w4)]; BIN40 [DRB1*0404 (DR4w14)]; SWEIG [DRB1*1101 (DR5w11)]; PITOUT [DRB1*0701 (DR7)]; KT3 [DRB1*0405 (DR4w15)]; Herluf [DRB1*1201 (DR5w12)]; H0301 [DRB1*1302 (DR6w19)]; OLL [DRB1*0802 (DR8w2)]; and HID [DRB1*0901 (DR9)]. In some instances, transfected fibroblasts were used: L466.1 [DRB 1*1501 (DR2w2b)]; TR81.19 [DRB3*0101 (DR52a)]; and L257.6 [DRB4*0101 (DRw53)]. Cells were maintained in vitro by culture in RPMI 1640 medium supplemented with 2 mM L-glutamine [GIBCO, Grand Island, N.Y.], 50 μM 2-ME, and 10% heat-inactivated FCS [Irvine Scientific, Santa Ana, Calif.]. Cells were also supplemented with 100 μg/ml of streptomycin and 100 U/ml of penicillin [Irvine Scientific]. Large quantities of cells were grown in spinner cultures.

[0055] Cells were lysed at a concentration of 10⁸ cells/ml in PBS containing 1% NP-40 [Fluka Biochemika, Buchs, Switzerland], 1 mM PMSF [CalBioChem, La Jolla, Calif.], 5 mM Na-orthovanadate, and 25 mM iodoacetamide [Sigma Chemical, St. Louis, Mo.]. The lysates were cleared of debris and nuclei by centrifugation at 10,000×g for 20 min.

Affinity Purification of HLA-DR Molecules

[0056] Class II molecules were purified by affinity chromatography as previously described (Sette, et al. J. Immunol. 142:35 (1989) and Gorga, et al. J. Biol. Chem. 262:16087 (1987)) using the mAb LB3.1 (anti HLA-DR) coupled to Sepharose 4B beads. Lysates were filtered through 0.8 and 0.4 μM filters and then passed over the anti-DR column, which were then washed with 15 column volumes of 10 mM Tris in 1% NP-40, PBS and 2-column volumes of PBS containing 0.4% n-octylglucoside. Finally, DR molecules were eluted with 50 mM diethylamine in 0.15 M NaCl containing 0.4% n-octylglucoside, pH 11.5. A {fraction (1/25)} volume of 2.0M Tris, pH 6.8, was added to the eluate to reduce the pH to ˜8.0, and then concentrated by centrifugation in Centriprep 30 concentrators at 2000 rpm (Amicon, Beverly, Mass.).

Class II Peptide-binding Assays

[0057] A panel of 13 different specific DR-peptide assays were utilized in the present study. These assays were chosen as representative of the most common DR alleles. FIG. 3 lists for each DR antigen, the representative allelic product utilized, the cell line utilized as a source of DR, and the radiolabled probe utilized in the assay. Purified human Class II molecules [5 to 500 nM] were incubated with various unlabeled peptide inhibitors and 1-10 nM ¹²⁵I-radiolabeled probe peptides for 48 h in PBS containing 5% DMSO in the presence of a protease inhibitor cocktail.

[0058] The radiolabeled probe peptides utilized for each assay are shown in FIG. 3. Radiolabeled peptides were iodinated using the chloramine-T method. Peptide inhibitors were typically tested at concentrations ranging from 120 μg/ml to 1.2 μg/ml. The data were then plotted and the dose yielding 50% inhibition (IC₅₀) was measured. In appropriate stoichiometric conditions, the IC₅₀ of an unlabeled test peptide to the purified DR is a reasonable approximation of the affinity of interaction (K_(D)). Peptides were tested in two to four completely independent experiments. The final concentrations of protease inhibitors were: 1 mM PMSF, 1.3 nM 1,10-phenanthroline, 73 μM pepstatin A, 8 mM EDTA, and 200 μM N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) (all protease inhibitors from CalBioChem, La Jolla, Calif.). Final detergent concentration in the incubation mixture was 0.05% Nonidet P-40. Assays were performed at pH 7.0 with the exception of DR3, which was performed at pH 4.5, and DRw53, which was performed at pH 5.0. The pH was adjusted as previously described (Sette, et al. J. Immunol. 148:844 (1992)).

[0059] Class II peptide complexes were separated from free peptide by gel filtration on TSK2000 columns (TosoHaas 16215, Montgomeryville, Pa.), and the fraction of bound peptide calculated as previously described (Sette, et al., (1989) supra). In preliminary experiments, the DR preparation was titered in the presence of fixed amounts of radiolabeled peptides to determine the concentration of Class II molecules necessary to bind 10-20% of the total radioactivity. All subsequent inhibition and direct binding assays were then performed using these Class II concentrations.

DRB1 Specificity of DR4w15, DR6w19, DR8w2, DR8w3. and DR9 assays

[0060] Because the antibody used for purification is HLA-DR α-chain specific, β1 molecules were not separated from β3 (and/or β4 and β5) molecules. Development and validation of assays in regard with DRβ chain specificity has been described for many of the DR alleles listed above. See, for example, Sette et al., J. Immunol. 151: 3163 (1993); Sidney et al., J. Immunol. 149: 2643 (1992); O'Sullivan et al., J. Immunol. 147: 2663 (1991); Valli et al., J. Clin. Invest. 91: 616(1993); Boitel et al., J. Immunol. 154: 3245 (1995); O'Sullivan et al., J Immunol. 145: 1799 (1990); and Wucherpfennig et al., J. Exp. Med. 179:279 (1994). DR4w15, DR6w19, DR8w2, DR8w3, and DR9 assays are described herein. The β chain specificity of these new assays is described hereafter.

[0061] DR4w15. The β4 product DRw53 is co-expressed with DR4w15 and the determination of the specificity of the DR4w15 binding assay is complicated in that the same radiolabeled ligand is used for both the DR4w15 and DRw53 binding assays. Since typically β1 chains are expressed at 5-10 fold higher levels than other β chains, and all binding assays are performed utilizing limiting DR amounts, it would be predicted that the dominant specificity detected in the assay would be DR4w15. To verify that this was indeed the case, the binding pattern of a panel of 58 different synthetic peptides in the putative DR4w15 specific assay was compared with that obtained in a DRw53 specific assay (which uses a DRw53 fibroblast as the source of Class II molecules). Two very distinct binding patterns were noted, and in several instances, a peptide bound to one DR molecule with high affinity, and did not bind to the other.

[0062] DR6w19. The DR6w19 assay utilizes as the source of Class II molecules the EBV transformed homozygous cell line H0301, which co-expresses DRB3*0301 (DR52a). While the radiolabeled ligand used in the DR6w19 assay is different than that used for the DR52a assay, the ligand is related (i.e., is a single substitution analog) to a high affinity DR52a binder. As was done in the case of DR4w15, the specificity of the assay was investigated by analyzing the binding capacity of a panel of naturally occurring peptides for DR6w19 and DR52a. The two assays demonstrated completely different binding specificities. For example, in terms of relative binding, TT 1272-1284 binds 63-fold better in the DR52a assay than in the DR6w19 assay. Conversely, the invariant chain peptide binds 189-fold better in the DR6w19 assay. In conclusion, these data demonstrated that the binding of the radiolabeled peptide 650.22 to purified Class II MHC from the H0301 cell line is specific for DR6w19.

[0063] DR8w2 and DR8w3. The B1 specificity of the DR8w2 and DR8w3 assays is clear in that no B3 (and/or B4 and B5) molecule is coexpressed.

[0064] DR9. The specificity of DR9 assay is inferred from previous studies which have shown that the TT 830-843 radiolabeled probe peptide does not bind to DRw53 molecules (Alexander, et al., Immunity 1:751 (1994)).

Results DR Binding Affinity of Antigenic Peptides Recognized by DR Restricted T Cells

[0065] To define a biologically relevant threshold of DR binding affinity, peptide binding affinities associated with 32 reported instances of DR restriction of a given T cell epitope were compiled. In approximately half of the cases, DR restriction was found to be associated with affinities of less than 100 nM. In almost all of the remaining cases, affinities were in the 100 -1000 nM range. Only in 1 out of 32 cases (3.1%) was DR restriction associated with an IC₅₀ of 1000 nM or greater. It was noted that this distribution of affinities differs from what was previously reported for HLA class I epitopes, where a vast majority of epitopes bound with IC₅₀ of 50 nM or less (Sette, et al., J. Immunol. 153: 5586 (1994)). This relatively lower affinity associated with class II restricted epitope interactions might explain why activation of class II restricted T cells in general requires more antigen relative to class I restricted T cells.

[0066] In conclusion, this analysis suggested that 1000 nM may be defined as an affinity threshold associated with immunogenicity in the context of DR molecules, and for this reason a suitable target for the studies described herein.

P1 and P6 Anchors are Necessary but not Sufficient for DRB1*0401 binding

[0067] Several independent studies have pointed to a crucial role in DRB1*0401 binding of a large aromatic or hydrophobic residue in position 1, near the N-terminus of the peptide, of a 9-residue core region (residues 1 through 9). In addition, an important role has been demonstrated for the residue in position six (P6) of this 9-residue core region. Short and/or hydrophobic residues were in general preferred in this position (O'Sullivan, et al., J. Immunol. 147:2663, 1991; Sette, et al., J. Immunol. 151:3163, 1993; Hammer, et al., Cell 74:197, 1993 and Marshall, et al., J. Immunol. 154:5927, 1995).

[0068] In the present set of experiments, a library of 384 peptides was analyzed for DRB 1*0401 binding capacity and screened for the presence of the P1-P6 motif (that is, F, W, Y, L, I, V or M in P1 and S, T, C, A, P, V, I, L or M in P6, with typically two or more residues flanking the N-terminal end of the P1 residue (SEQ ID NO: 1). This set of 384 peptides contained a total of 80 DRB1*0401 binders, of which 27 were good binders [IC₅₀ of 100 nM or less], and 53 were intermediate binders [IC₅₀ in the 100-1000 nM range]. Seventy-seven out of the 80 (96%) DRB1*0401 binders carried the P1-P6 motif. However, it should be noted that most non-DRB1*0401 binding peptides also contained the P1-P6 motif. Of384 peptides included in the database, only 125 were “P1-P6 negative.” Only three of them (6%) bound appreciably to purified DRB1*0401 as opposed to 77/259 (30%) of the “P1-P6 positive” peptides. Therefore, these results demonstrate that presence of suitable P1 and P6 anchors are necessary but not sufficient for DRB1*0401 binding.

A Detailed Map of DRB1*0401 Peptide Interactions

[0069] Next, the P1-P6 core region of all peptides in the database were aligned. Then, in analogy with what the strategy previously utilized to detail peptide class I interactions, the average binding affinity of peptides carrying a particular residue, relative to peptides with any other residue, were calculated for each position. Following this method a table of average relative binding (ARB) values was compiled (FIG. 1). This table also represents a map of the positive or negative effect of each of the 20 naturally occurring amino acids on DRB1*0401 binding capacity when occupying a particular position, relative to the main P1-P6 anchors.

[0070] Variations in ARB values greater than four fold (ARB ≧4 or ≦0.25) were considered significant and indicative of secondary effects of a given residue on DR-peptide interactions. Most secondary effects, positive or negative, were associated with positions 4, 7, and 9. These positions correspond to secondary anchors engaging shallow pockets on the DR molecule. In addition, significant positive secondary effects were detected for M in position 2(ARB=12.8), T in position 3 (ARB=4.34) and I in position 5 (ARB=4.4).

Development of a DRB1*0401 Specific Algorithm

[0071] The ARB table was next utilized to develop a DRB1*0401 specific algorithm. In order to predict DR1*0401 binding propensity, each P1-P6 sequence was scored by multiplying, for each position, the ARB value of the corresponding amino acid. According to this procedure, a numerical “algorithm score” was derived. If multiple P1-P6 alignments were possible for a specific peptide, algorithm scores were calculated for each alignment and the best score was selected. The efficacy of this method in predicting DRB1*0401 binding capacity is shown in FIG. 4A.

[0072] Considering only peptides with algorithm scores above -17.00 narrowed the set of predicted peptides to 156. This set still contained 72 out of 80 (90%) of the total high or intermediate DR binders. Raising the algorithm cut-off score to −16.44 further reduced the set selected to 107 peptides, but still allowed identification of 60 of the 80 (75%) DRB1*0401 binding peptides. Thus, the algorithm significantly reduced the number of peptides required for analysis while only slightly reducing the number of DRB1*0401 binding peptides in the set.

Blind Test of the Predictive Power of the DRB1*0401 Specific Algorithm

[0073] To verify that the predictive capacity of the algorithm was not merely a reflection of having utilized the same data set to test and define the algorithm itself, the efficacy of the algorithm was further assessed in a blind prediction test. For this purpose, data from an independent set of 50 peptides, where binding affinities were known but had not been utilized in the derivation of the algorithm, was utilized. As shown in FIG. 4B, the algorithm was effective in predicting DRB1*0401 binding capacity of this independent peptide set. An algorithm score of greater than −17.00 identified a total 18 peptides. This set contained 3/3 (100%) of all good binders, and {fraction (8/11)} (70%) of all intermediate binders in the entire test set of 50 peptides. Increasing the cut-off value to-16.44, identified nine peptides. Seven of them (78%) were either good or intermediate binders. This set contained 7 of the 14 (50%) binders contained in the blind prediction peptide set. In conclusion, these data support the validity of the DRB1*0401 specific algorithm described above.

Detailed Maps of DRB1*0101, and DRB1*0701 Peptide Binding Specificities

[0074] Next, the DR1 and DR7 binding capacity of the same set of 384 peptides utilized to define the DRB1*0401 algorithm was measured. It was found that this set contained 120 DR1 binders and 59 DR7 binders. A total of 158 peptides were capable of binding either DR1, DR4w4 or DR7. A large fraction of them (73/158; 46%) were also degenerate binders, having the capacity to bind two or more of the three alleles thus far considered. Analysis of individual peptide sequences revealed that more than 90% of the DR1 or DR7 good and intermediate binders and 72 out of 73 (99%) degenerate binders carried the P1-P6 motif. This analysis suggests that P1-P6 based algorithms might be utilized to effectively predict degenerate DR binders.

[0075] In analogy with what was described above for DRB1*0401 molecules, specific algorithms were designed for the DR1 and DR7 molecules. FIGS. 2A and 2B detail the molecule specific maps defined according to this method.

[0076] As in the case of DRB1*0401, most secondary effects associated with DR1 and DR7 were concentrated in positions 4, 7 and 9. Position 4 was especially prominent in the case of DR1, while position 7 was the most prominent secondary anchor for DR7. Specific algorithms were developed based on these maps, and the cut-off values necessary to predict 75% or 90% of the binders were −19.32 and −20.28 for DR1, and −20.91 and −21.63 for DR7. Depending on the particular molecule or cut off value selected, 40% to 60% of the predicted peptides were in fact good or intermediate binders.

Development of a DR 1-4-7 Combined Algorithm

[0077] To examine whether a combined algorithm would allow prediction of degenerate binders, the sequences of the 384 peptides in the database were concurrently analyzed using the DR1, DR4, and DR7 algorithms. Using the 75% cut off, it was found that 100 of the 384 peptides were predicted to bind either two or three of the molecules considered. The 75% cut off also identified 59 of 73 (81%) peptides which were in fact capable of degenerate binding (defined as the capacity to bind to more than one of the DR1, DR4w4, or DR7 molecules) (FIG. 5).

Definition of a Target set of DR Specificities, Representative of the World Population

[0078] The data presented in the preceding sections illustrates how peptides capable of binding multiple DR alleles can be identified by the combined use of DR1, DR4w4, and DR7 (1-4-7) algorithms. It was next determined whether the peptides exhibiting degenerate 1-4-7 binding behavior would also bind other common DR types as well. As a first step, a set of target DR types representative of a large (>80%) fraction of the world population, irrespective of ethnicity, was defined. Seven additional DR antigens were considered. The estimated frequency in various ethnicities, according to the 11th HLA workshop (Imanishi et al., in HLA 1991: Proceedings of the eleventh international histocompatibility workshop and conference, Tsuji et al., eds., Oxford University Press, Tokyo, p. 1065, 1991) is shown for each antigen (including DR1, DR4w4, and DR7) in FIG. 6. The main subtypes identified that are associated with each antigen are also shown.

[0079] For the purpose of measuring peptide binding affinity to various DR molecules, one representative subtype for each DR antigen was chosen (FIG. 3). It should be noted that for most antigens, either one subtype is by far the most abundant, or alternatively, a significant degree of similarity in the binding pattern displayed by the different most abundant subtypes of each DR antigen is likely to exist (FIG. 3). One exception to this general trend is represented by the DR4 antigen, for which significant differences in peptide specificity between the DRB1*0401 and DRB1*0405 molecules have been reported. Since both alleles are quite frequent (in Caucasians and Orientals, respectively) both DRB1*0401 and DRB1*0405 were included in the set of representative DR binding assays.

[0080] As shown in FIG. 3, the set of representative assays is mostly focused on allelic products of the DRB1 gene. DRB1 molecules appear to be the most abundantly expressed, serve as the dominant restricting element of most human class II responses analyzed thus far, and accurate methods for their serologic and DNA typing are most readily available. However, an assay representative of the DRB5 molecule DR2w2a was also included in the panel of representative assays. These molecules can similarly serve as a functional restriction element, and their peptide binding specificity has been shown to have certain similarities to the specificity of several common DRB1 allelic products and it is also relatively common in most major ethnicities (average greater than 23%).

A General Strategy for Prediction of DR-degenerate Binders

[0081] To test whether the 1-4-7 combined algorithm would also predict degenerate binding to other common DR types, the capacity to bind the panel of HLA-DR molecules identified in FIG. 3 was determined for three different groups of synthetic peptides. The three different peptide sets were: A) 34 peptides which did not score positive in the combined 1-4-7 algorithm (non-predictions), B) 24 peptides which did score positive for the 1-4-7 algorithm at the 75% cut off level but had been found upon actual testing not to be degenerate 1-4-7 binders (“wrong” predictions), and C) 29 peptides which scored positive in the 1-4-7 algorithm and also proved upon experimental testing to be actual 1-4-7 degenerate binders (correct predictions).

[0082] In the set of “non-predictions” peptides (FIG. 7A) only 3 out of 34 (9%) bound at least two of the DR1, DR4w4, or DR7 molecules. Two (1136.04 and 1136.29) out of these three peptides were also crossreactive, binding at least four additional DR types. Peptides from the “wrong predictions” set, which by definition bound at most only one molecule from amongst the DR1, DR4w4 or DR7 molecules, were also poorly cross reactive for other DR types. Specifically, only two peptides (1136.22 and 1188.35) bound a total of three DR molecules. No peptide bound four or more of the DR molecules tested.

[0083] These results are in striking contrast with those obtained for the set of peptides predicted by the use of the combined 1-4-7 algorithm, and then experimentally confirmed to be degenerate DR 1-4-7 binders. In this case, sixteen out of 29 peptides (55.2%) bound five or more different DR types. Four of these (1188.16, 1188.32, 1188.34 and F107.09) were remarkably degenerate, having the capacity to bind nine out of the 11 DR molecules tested.

[0084] In conclusion, these results suggest that a strategy based on the sequential use of a combined DR 1-4-7 algorithm and quantitative DR 1-4-7 binding assays can be utilized to identify broadly crossreactive DR binding peptides. Specifically, 16 out of 18 (88.9%) of the peptides found to bind five or more DR types were contained within the set of peptides predicted and experimentally confirmed to be DR 1-4-7 degenerate binders.

Definition of the HLA-DR 1-4-7 Supertype

[0085] The data presented above also suggested that several common DR types are characterized by largely overlapping peptide binding repertoires. This issue was examined in more detail by contrasting the overall binding pattern of the thirty-two DR 1-4-7 degenerate peptides with that of 67 non-DR 1-4-7 degenerate peptides (FIG. 8 and FIG. 9). As noted above, a large fraction of the DR 1-4-7 degenerate binders also bound certain other common DR types with high frequency. Specifically, sixteen (50%) bound DR2w2a, 18 (56%) DR6w19, 18 (56%) DR2w2b, and 20 (62%) DR9. In all cases, the corresponding frequency of binding in the non-1-4-7 degenerate peptide set was much lower, and in the 3% to 24% range. Significant, albeit lower frequencies of cross reactivity for the degenerate 1-4-7 binders were noted also in the context of DR4w15, DR5w11, and DR8w2 (in the 28 to 37% range). Finally, negligible levels of cross reactivity were observed in the case of DR3 and DR5w12 and DR53.

[0086] In conclusion, these data demonstrate that a large set of DR molecules encompassing DR1, DR4w4, DR7, DR2w2a, DR2w2b, DR9 and DR6w19 is characterized by largely overlapping peptide binding repertoires. Furthermore, peptides having the capacity to bind multiple molecules from this set can be easily identified using the combination of DR 1-4-7 algorithms and DR 1-4-7 peptide binding assays.

Discussion

[0087] The peptide binding specificity of HLA DR molecules representative of DR types common among the worldwide population were analyzed. Detailed maps of secondary interactions have been derived for three of them (DR1, DR4w4, and DR7). Furthermore, a set of at least seven different DR types share overlapping peptide binding repertoires, and consequently broadly degenerate HLA DR binding peptides are a relatively common occurrence. This study also describes computerized procedures which should greatly assist in the task of identification of such degenerate peptides.

[0088] These studies illustrate how the vast majority of the peptides binding with good affinity to DR4w4, DR1, DR7 and most of the other DR types analyzed in the current study (data not shown), are all characterized by a P1-P6 motif consistent with the one originally proposed by O'Sullivan, et al. supra. Crystallographic analysis of DR1-peptide complexes (Brown et al., Nature 364: 33 (1993); Stem et al., Nature 368: 215 (1994)) revealed that the residues occupying these positions engage two complementary pockets on the DR1 molecule, with the P1 position corresponding to the most crucial anchor residue and the deepest hydrophobic pocket. The present analysis also illustrates how other “secondary anchor” positions drastically influence in an molecule-specific manner peptide binding capacity. Position 4 was found to be particularly crucial for DR1 binding, position 9 for DR4w4, and position 7 for DR7. These data are consistent with previous results which originally described such allele-specific anchors, and with crystallographic data which illustrates how these residues engage shallow pockets on the DR molecule.

[0089] The present studies also illustrate how an approach based on sequence alignment and calculation of average relative binding values of large peptide libraries allows definition of quantitative algorithms to predict binding capacity. These results also illustrate how the combined use of the molecule specific 1-4-7 algorithms can be of aid in identifying broadly degenerate DR binding peptides.

[0090] The data presented herein suggest that a group of common DR alleles, including at least DR1, DR2w2a, DR2w2b, DR4w4, DR6w19, DR7 nd DR9 share a largely overlapping peptide binding repertoire. Degenerate peptide binding to multiple DR molecules, and recognition of the same epitope in the context of multiple DR types was described previously. The present study provides a classification of molecules belonging to a main HLA-DR supertype (DR 1-4-7-like) which includes DR1, DR2w2a, DR2w2b, DR4w4, DR7, DR9, and DR6w19. On the basis of the data presented herein, two additional groups of molecules may exist. The first group encodes for molecules (DR4w15, DR8w2, DR5w11) with reduced although not insignificant overlap with the 1-4-7-like supertype. The second group of molecules (DR5w12, DR3w17, and DRw53) clearly has little repertoire association with the DR 1-4-7 supertype. In this context Hammer, et al., supra, noted that good DR5w11 binding peptides are frequently characterized by a positively charged P6 anchor, which would be poorly compatible with the herein proposed DR 1-4-7 supermotif. Also Sidney, et al., J. Immunol. 149: 2643 (1992) proposed that DR3 w17 binds a set of peptides largely distinct from those bound by other common DR types. Future studies will have to determine whether any of the molecules listed above can be grouped in additional DR supertypes.

[0091] There are similarities and differences between the HLA DR supertype described herein and HLA class I supermotifs. Class I supermotifs are clear-cut and, as a rule, non-overlapping. Four of them have been described in detail and are approximately equally frequent amongst the worldwide population. By contrast, the repertoire defining the HLA DR supertype herein described is not clear-cut and overlaps, at least in part, with the repertoire of other molecules. It also appears on the basis of the data presented in FIGS. 3 and 6, even if other DR supertypes exist, the DR 1-4-7 is going to be by far the most abundantly represented worldwide. The DR 1-4-7 supertype would allow coverage in the 50 to 80% range, depending on the ethnicities considered. It is thus possible that broad and not ethnically biased population coverage could be achieved by considering a very limited number of peptide binding specificities.

[0092] These data are relevant for the development of epitope-based vaccines. Class II restricted HTL have been implicated in protection from, and termination of many important diseases. Inclusion of well defined class II epitopes in prophylactic or therapeutic vaccines may allow focusing the immune response towards conserved or subdominant epitopes, and avoid suppressive determinants.

[0093] Based on the results present above, the sequences of various antigens of interest were scanned for the presence of the DR 1-4-7 motifs. Peptides identified using this approach, as demonstrated above, are predicted to be broadly cross reactive class II restricted T cell epitopes. FIG. 10 presents a listing of such peptides derived from HBV, HCV, HIV and Plasmodium falciparum (Pf). A total of 146 peptides were identified: 35 from HBV, 16 from HCV, 50 from HIV, and 45 from Pf. Standard conservancy criteria were employed in applicable cases. Also, peptides from other organisms having the 1-4-7 motif are set forth in FIG. 11.

[0094] The above example is provided to illustrate the invention but not to limit its scope. Other variants of the invention will be readily apparent to one of ordinary skill in the art. All publications, patents, and patent applications cited herein are hereby incorporated by reference for all purposes 

What is claimed is:
 1. A method for inducing or detecting a T-cell response, which comprises (a) providing a peptide that bears a DR motif corresponding to a Class II HLA molecule; (b) testing the peptide for binding affinity to a Class II HLA DR molecule, whereupon an IC₅₀ binding affinity is determined; (c) comparing the binding affinity of the peptide to an IC₅₀ affinity threshold of 1,000 nM; (d) identifying as a peptide suitable for administration as an immunogen a peptide that binds said Class II HLA DR molecule at a binding affinity of less than 1,000 nM; and (e) contacting a Class II HLA DR molecule with a peptide identified by the steps (a) to (d), wherein a T-cell response is induced or detected.
 2. The method of claim 1, wherein the Class II HLA DR molecule is contacted with the peptide in an antigen presenting cell.
 3. The method of claim 1, wherein the Class II HLA DR molecule is contacted with the peptide ex vivo.
 4. The method of claim 1, wherein the class II HLA DR molecule is selected from the group consisting of DR1, DR2w2b, DR3w17, DR4w4, DR4w15, DR7, DR8w2, DR9, DR5w11, DR5w12, DR6w19, and DR2w2a.
 5. The method of claim 2, wherein the antigen presenting cell is a dendritic cell.
 6. The method of claim 1, wherein the motif consists of Y, F, W, L, I, V, or M in the first position and S, T, C, A, P, V, I, L, or M in the sixth position of a 9-residue core region of said peptide.
 7. The method of claim 1, wherein the peptide consists of a sequence selected from the group consisting of SEQ ID NO: 40-274.
 8. The method of claim 1, wherein the peptide is less than 50 amino acids in length.
 9. The method of claim 8, wherein the peptide is about 10 to about 30 amino acids in length.
 10. The method of claim 9, wherein the peptide is about 15 to about 20 amino acids in length.
 11. The method of claim 1, wherein the peptide is encoded by a nucleic acid.
 12. The method of claim 2, wherein the method further comprises contacting the antigen presenting cell with a second peptide, wherein the second peptide is capable of inducing a CTL response.
 13. The method of claim 12, wherein the second peptide is linked to the peptide that bears a motif corresponding to the class II HLA DR molecule. 